1. Field of the Invention
The invention is directed to a non-mechanical printer means, and particularly to an electrophotographic printer means wherein an image is generated on a recording medium from individual points by use of a plurality of light-emitting elements of an optical character generator.
2. Description of the Prior Art
Printer means operating on the principle of electrophotography contain optical character generators. These optical character generators have the job of converting the print information existing in the form of electronic data into an optical image with which a photoconductive layer, for example a photoconductor drum, is then exposed. The exposed image is then developed in a known way and, for example, transferred onto paper.
Optical character generators that are constructed in line-like fashion are advantageous since these operate without mechanical movement. Given this type of character generation, a separate light source must be present for every point which is to be imaged within a line. The number of light sources is thereby extremely high (several 1000). All light sources must expose the photoconductive layer with the same light energy so that an optimum quality of the print image is achieved. Light-emitting diodes are preferably utilized as light sources. These have the disadvantageous property that the light yield of the individual light-emitting diodes fluctuates greatly. In order to meet high quality demands, the individual light-emitting diodes must be corrected in a suitable way, i.e. must be individually driven, so that the emitted light energy per exposure point is approximately the same, even given differing light yield.
Such correction methods are notoriously known. Thus, German Published application No. 32 30 226 corresponding to U.S. Pat. No. 4,455,562, incorporated herein by reference, discloses a method and an apparatus for the control of a light-emitting diode field in a non-contacting printer, for example an electrophotographic printer, wherein the light-emitter diodes are employed for generating an image on a photoconductor.
Every individual light-emitting diode is thus previously balanced once before placing the printer means into operation and is digitally stored in stages in a read-only memory (PROM).
Further, German Published Application No. 34 22 907, corresponding to U.S. Pat. No. 4,596,995, incorporated herein by reference, discloses a dot matrix printer. The dot matrix printer has a light-emitting diode arrangement print head comprising an arrangement of light-emitting diodes, driver stages for independent operation of the light-emitting diodes, and time control circuits for the control of the turn-on times or of the turn-off times of the light-emitting diodes on the basis of the drive of the driver stages in accordance with the image data and correction data which are stored in a memory. The light-emitting diodes can be divided into blocks which can be successively driven. Deviations of mechanical and electrical properties of the light-emitting diodes are thus corrected, so that the generation of an image having high resolution and of a half-tone image is possible.
Here too, only the correction of the manufacturing related fluctuations of the light hield of the light-emitting diodes is provided.
The light yield of light-emitting diodes, however, does not fluctuate only during manufacture but also additionally changes in an unpredictable way given increasing aging. Added thereto is a high temperature dependency of the light power.
If character generators comprising light-emitting diodes are to be utilized in high-performance electrophotographic printer means which, for example, print between 50 and 140 sheets per minute, then a one-time balancing in the manufacture of the printer means is inadequate.